Ever try flying clubs? Generally they have some decent rates for planes. Be wise to do some flying with a credible instructor towards the commercial since they can get you ready for being proficient in the commercial test standards.
You'll obviously need a multi ticket as well which is very expensive. There is a guy in Oshkosh who has the cheapest multi. He does not allow renting solo yet even with the plane and instructor it is still way cheaper than just renting a multi engine airplane anywhere else.
If you're thinking airlines down the road I would recommend getting your CFI. It's arguably one of the best ways to build time consistently all year long. Plus you can make some $ doing it. Just my .02